Elastomers are conventionally reinforced with particulate reinforcing fillers such as, for example, carbon black and sometimes synthetic amorphous silica, usually precipitated silica aggregates.
Such synthetic silica aggregates are normally highly hydrophilic in nature and have a strong tendency to form agglomerates of the aggregates.
Usually a coupling agent is also added to the rubber composition to aid in coupling the silica to the one or more of the diene-based elastomers contained in the rubber composition. Alternately, an alkoxysilane hydrophobating agent for the silica may also be added to the rubber composition or the silica may be pre-treated with such coupling agent and/or alkoxysilane hydrophobating agent.
In practice, the rubber composition is usually subjected to several sequential high shear, high temperature, mixing steps in which the silica agglomerates are substantially physically broken down and thereby substantially reduced to the aforesaid basic silica aggregates.
Masterbatches, or pre-formed mixtures, of carbon black and rubber have sometimes been suggested and/or used for the preparation of various rubber compositions. Often such masterbatches are prepared by introducing water slurries of carbon black to an aqueous rubber latex, followed by coagulating and recovering the resulting composite for use in blending with a respective rubber composition.
Masterbatches of silica and rubber have also been suggested for similar purposes where the masterbatch is created by blending silica and an aqueous rubber latex, followed by recovering the resulting blend of silica and rubber.
However, because the precipitated silica aggregates are typically hydrophilic in nature and therefore have an affinity to water, the silica aggregates typically become more associated with the water of the latex than with the rubber itself. Accordingly, it is considered herein that it is difficult to adequately disperse such hydrophilic silica within a rubber by introducing the silica to the rubber in its aqueous latex form.
Accordingly, various methods have been offered to more efficiently provide dispersions of silica in a rubber latex.
For example, it has been proposed to incorporate silica into a rubber latex by
(A) treating a silica with coupling agent in a aqueous suspension to form a compatibilized silica slurry, said coupling agent being an organosilane silicon compound having the capability of chemically reacting with the surface of the silica to bond the coupling agent thereto, and
(B) contacting the rubber latex with the treated silica slurry whereby the silica is substantially uniformly distributed through the latex. For example, see U.S. Pat. No. 5,763,388.
Historically, precipitated silica aggregates are sometimes formed by acid-coagulating a silicate from a basic solution thereof, such as, and only as an example, treating sodium silicate, formed from silica and sodium hydroxide, with sulfuric acid, in the presence of a suitable electrolyte, under controlled conditions such as for example controlled temperature and controlled sequential order and rate of addition of materials. It is envisioned herein that colloidal elemental particles of silica are thereby formed which readily aggregate to from colloidal silica aggregates thereof and which are thereafter recovered from such colloidal mixture by precipitation of the silica aggregates therefrom to form a filtercake thereof which is washed with water, which might also include an electrolyte, and finished by drying techniques.
It is understood that such colloidal silica may negatively charged or positively charged, depending somewhat upon the specific manufacturing method used to form the colloidal silica.
Such colloidal silica, whether in a form of said formative elemental particles of silica and/or silica aggregates thereof, preferably in a form of colloidal silica aggregates, and prior to said recovery thereof by precipitation from said colloidal mixture, is referred to herein as “colloidal silica” which may be in an unprecipitated colloidal/dispersion state of aggregates of primary silica particles having hydroxyl groups (e.g. silanol groups) on their surfaces.
For this invention, it is desired to prepare a composite of silica and diene-based rubber by mixing an aqueous dispersion of well dispersed, negatively charged colloidal silica with a diene-based rubber latex and discrete domains (droplets) of a negatively charged oil emulsion of an organosilane followed by co-coagulating the mixture with the aid of a suitable coagulant to form a silica/rubber/silane composite.
By such method it is envisioned that a novel composite of silica and diene-based rubber is formed with relatively homogeneously dispersed silica aggregates therein and with a minimum of premature silane hydrolysis and network formation in the elastomer host.
In the description of this invention, the term “phr” where used herein, and according to conventional practice, refers to “parts of a respective material per 100 parts by weight of rubber, or elastomer”.
In the description of this invention, the terms “rubber” and “elastomer” if used herein, may be used interchangeably, unless otherwise prescribed. The terms “rubber composition”, “compounded rubber” and “rubber compound”, if used herein, are used interchangeably to refer to “rubber which has been blended or mixed with various ingredients and materials” and such terms are well known to those having skill in the rubber mixing or rubber compounding art.